To prevent counterfeiting of paper sheets, a technology in which a motion thread is used is known in the art. The motion thread is a type of a thread that is used in paper sheets. The motion thread is sometimes simply referred to as motion, or, because the motion thread enhances security of the paper sheets, it is also referred to as a security ribbon.
The motion thread is formed by arranging a micro-lens, such as, a lenticular lens, on a plurality of micro-images, called icons, via an optical spacer. Details of a structure and optical properties of a motion thread are disclosed in, for example, U.S. Pat. No. 7,333,268.
The motion thread is used by embedding in the paper sheet so as to form, for example, a small strip region on the paper sheet. When the paper sheet is tilted while looking at a pattern on a motion thread part, the pattern seems to move with the movement of the paper sheet. With this feature, it can be judged under a visible light whether the paper sheet is a counterfeit or not. Therefore, a technology whereby it can be judged speedily and accurately whether a paper sheet includes a motion thread is desirable for the paper sheet recognition apparatus.
The pattern appeared the motion thread part differs according to structures, etc., of the icons and the micro-lens. Meanwhile, apart from cases where the same pattern appears to be moving when the paper sheet is being tilted, there are cases where different patterns appear depending on an angle of tilt of the paper sheet.
To judge the presence ox absence of the motion thread in the paper sheet, a method can be conceived in which a recognition technology relating to holograms or color shift inks of which patterns and color vary with a viewing angle is used, however, the principle thereof differs from that of the motion thread. For example, the conventional technology for recognizing the holograms, etc., is disclosed in Japanese Patent Application Laid-open No. 2007-213210. Specifically, a paper sheet is irradiated with light using a single phototransmitter and a light reflected by a surface of the paper sheet is received by a plurality of photoreceivers and analyzed, and authenticity of the paper sheet is judged based on the result of the analysis. For example, multiple diffraction reflection lights produced when the hologram is irradiated with light are received by the photoreceivers and analyzed to judge the authenticity of the paper sheet.
Because the above-described conventional technology is relating to the holograms, etc., a recognition device and a recognition method adapted to the conventional technology cannot be used as it is for recognition of the motion thread. Specifically, an arrangement of an optical source and sensors of the device and a data processing method need to be optimized to cope with the features of the motion thread.
In the above-described conventional technology, multiple light-receiving elements are required, which leads to an increase in a size of the device as well as a manufacturing cost. Specifically, the light receiving elements are required in a number equal to the number of the reflected lights to be measured, and a scale of circuitry also increases for processing signals measured by each light receiving element, and as a result, the manufacturing cost increases. The cost further increases when an optical system, such as, an SLA (Selfoc Lens Array), is required between the light receiving element and the paper sheet. Moreover, the size of the device increases due to an increase in the number of constituent elements, such as, the light receiving elements, and the optical system, and an enlargement in the scale of circuitry.